In modern automobiles, precise control of air-fuel ratio (A/F) to a stoichiometric value is necessary for optimum performance of the three-way catalytic converter (TWC) and consequent minimization of exhaust emissions. A/F control generally consists of two components: a feedback portion in which a signal related to A/F from an exhaust gas oxygen (EGO) sensor is fed back through a digital controller to regulate the fuel injection pulse width, and a feed forward portion in which injector fuel flow is controlled in response to a signal from an air flow meter. The feedback, or closed-loop portion of the control system, is fully effective only under steady state conditions and when the EGO sensor has attained the proper operating temperature. The open-loop, or feed forward portion of the control system, is particularly important when the engine is cold (before the closed-loop A/F control is operational) and during transient operation when inherent delays in the closed-loop A/F feedback system inhibits good control. Typically, the signal from the air flow meter is used to generate an estimate of instantaneous manifold pressure. This estimate along with engine speed and, potentially, other engine variables, such as EGR, vapor purge, etc., defines the flow rate of air into the engine cylinders from the manifold. Finally, cylinder air charge is determined by integrating the cylinder flow rate of air over the time required for the engine to complete one intake stroke. The cylinder air charge divided by the stoichiometric A/F ratio is the amount of fuel required for operation at stoichiometry and is used to calculate the appropriate injector pulse width.
The inventors herein have recognized two deficiencies with the conventional scheme. First, in order to provide an accurate dynamic estimate of the air flow entering the engine, it is essential to modify the air meter signal to account for the dynamic characteristics of the meter itself. The signal from the air meter does not respond instantaneously to changes in air flow. Hence, the conventional method of calculating manifold pressure and thus cylinder air charge on the basis of this uncorrected signal under estimates the amount of air in the intake manifold when the true air flow increases, and over estimates it in the case of a decrease in true air flow. Secondly, known methods of accounting for air meter dynamics require differentiating the electronic signal from the air meter. This approach results in undesirable noise amplification.